Impurities acceptor

In n type semiconductors, electrons are tire majority carriers. Holes will also be present tlirough accidental incoriioration of acceptor impurities or, more importantly, tlirough tlie intentional creation of electron-hole pairs. Holes in n type and electrons in p type semiconductors are minority carriers. 

Ga( Zn)silicon The two-electron acceptor impurity of Zn is present in silicon only in the form of neutral (ZnO) or doubly ionized Zn centers depending on the Fermi-level position. Broadening of the spectra corresponding to the above centers indicates that the local symmetry of these centers is not cubic... 

For example, Heckelsberg and his associates (S3) have discovered that the introduction of a donor impurity (Al) into ZnO increases the reaction rate, while the addition of an acceptor impurity (Li) retards the reaction. 

Molinari and Parravano (30) have also noted that the incorporation of a donor impurity (Al, Ga) into ZnO specimens promotes the exchange reaction, while an acceptor impurity (Li) slows it down. 

The same authors (41) working with specimens of silica gel observed a positive photocatalytic effect in the course of the hydrogen-deuterium exchange reaction. In this case the introduction of an acceptor impurity into a catalyst enhanced the action of irradiation. 

The introduction of an impurity into a crystal causes a displacement of the Fermi level both inside the crystal and, generally speaking, at its surface [in this case the Fermi level is displaced in the same direction both at the surface and in the bulk of the crystal, see reference (1) ]. This results, according to (63) and (5), in a change of g0. A donor impurity displaces the Fermi level upward, while an acceptor impurity shifts it in the opposite direction. The same impurity exerts diametrically opposite influences on the catalytic activity in acceptor and donor reactions. 

The great majority of experimental data (see Section III.A) indicate that the hydrogen-deuterium exchange reaction belongs to the class of acceptor reactions (i.e., reactions that are accelerated by electrons and decelerated by holes). This means that the experimenter, as a rule, remains on the acceptor branch of the thick curve in Fig. 8a, on which the chemisorbed hydrogen and deuterium atoms act as donors. Here a donor impurity must enhance the catalytic activity, while an acceptor impurity must decrease it. This is what actually occurs, as we have already seen (see Section III.A). 

Suppose now that the introduction of an acceptor impurity (increase of ev and es ) brings us from the point A to the point C (Fig. 9). This involves an increase in K, as seen from Fig. 9. This is in agreement with the results obtained by the same authors (41), who observed an increase in the photocatalytic effect on silica gel when acceptor impurities were added to the catalyst, and also with the data of Lunsford and Leland (42) who found that the effect was enhanced on MgO with increasing concentration of V-centers (acceptors). 

Thus, Romero-Rossi and Stone (11) have found that the effect is enhanced on ZnO when an acceptor impurity (Li) is introduced into the specimen. The increase of the effect on Cu20 upon the introduction of acceptor impurities (S and Sb) has also been observed by Ritchey and Calvert (58). The addition of a donor (Cr) to ZnO, as reported (11), lowers the magnitude of the effect. 

Suppose that the introduction of an acceptor impurity (increase of v and, -) transfers us from point A to point C, while in the case of addition of a donor impurity (decrease of v- and es ) we are transferred from the point A to B in Fig. 9. In this case, as is evident from Fig. 9, the acceptor impurity will enhance, and the donor impurity, weaken the photocatalytic effect. This is what has been observed by Romero-Rossi and Stone (11) on ZnO specimens and by Ritchey and Calvert (58) on Cu204 using Li, S, and Sb as acceptor impurities and Cr as a donor impurity. 

Note that the impurities exert opposite influences on the reaction in the dark. The oxidation of CO, like any acceptor reaction, is retarded by acceptor impurities (increase of tB ) and accelerated by donor impurities (decrease of e3 ). As a matter of fact, according to Parravano s data (61)... 

Stephens and co-workers (69) have found that the preheating of CdS specimens in an atmosphere of nitrogen (the purpose of preheating is to enrich a CdS specimen in an acceptor impurity) reduced the catalyst activity in relation to the photooxidation of water. 

This chapter is devoted to the energetics and kinetics of the incorporation of hydrogen into the simplest and most studied of its possible hosts, crystalline silicon of high perfection containing known concentrations of shallow donor or acceptor impurities. It undertakes to review what has been learned from experiments about the phenomenological parameters... 

Fig. 2-26. Localized electron levels of lattice defects and impurities in metal oxides Mi = interstitial metal ion Vm = metal ion vacancy V = oxide ion vacancy D = donor impurity A = acceptor impurity.

The dependence of the reaction rates qa and gg on the position of the Fermi level, as given by Equations (13), (14), and (15), is schematically depicted in Figs. 19a and 19b, respectively. We see that the lowering of the Fermi level retards dehydrogenation and accelerates dehydration. This gives us a key to the regulation of catalyst selectivity. Factors which bring down the Fermi level (for example, an acceptor impurity introduced into... 

It follows from Equations (30), (32), and (33a,b) that acceptor reactions are accelerated by a donor impurity and retarded by an acceptor impurity, and vice versa in the case of donor reactions. Thus, a given impurity on a given catalyst may be a promoter for one reaction and a poison for another. This is frequently observed in practice. For example, the addition of LijO to ZnO promotes the reaction of dissociation of NjO (71) and at the same time poisons the reaction of oxidation of CO (60). 

Similarly, one can derive an equation for a p-type semiconductor, where the distance between the Fermi energy level and the valence band is a logarithmic function of acceptor impurity concentration. As the acceptor impurity increases so too does the hole concentration in valence band, with the Fermi level moving closer to the valence band. 

Again, if there are several donor or acceptor impurities (or defects), designated by = 1, 2,..., then... 

The numerous defects inherent in organic polymers creates the donor or acceptor impurity levels. The low drift mobilities of the order 10-7-10-12 m2 V-1 s"1 lead to the paradoxical situation where the length of the free travel distance for the charge carrier becomes less than the size of the separate molecule links. So the hopping or activated models are the most acceptable ones for polymers in such circumstances. 

A certain amount of selenium may be considered as an acceptor impurity to copper oxide since the Fermi level of copper oxide catalyst is lowered or its p-typeness is increased. This agrees with the observation of Margolis (16). 

The presence of the region of weak dependence of the conductivity of alloyed semiconductors on temperature can be explained by tunneling of electrons from one impurity centre to another, unoccupied centre. The necessary condition of the impurity conductivity is the partial filling of the impurity levels. At low temperatures this conduction can be maintained only by semiconductor compensation, i.e. by the simultaneous presence of donor and acceptor impurities. In the case, for instance, of the n-type semiconduc-... 

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